no worries :)

it is recommended to use `double` over `mock` and `stub`
http://pragprog.com/book/achbd/the-rspec-book
and they raise nice errors when called with an unknown method (instead of `method_not_found` which you get on an `Object`)

For example, this should be close to what you want:
```
let(:disk) { double(Bosh::Agent::Platform::Linux::Disk) }